CSV question export and import
Supersedes !7 (closed), !8 (closed).
This feature lets the user export and import questions from a CSV file if the file meets the requirements of the specification.
Changes:
- Branch & code clean up
- Rebase on master
- Squash related commits authored by the same developer
- Remove commits whose code has been reverted later on
- Fix typos
- Fix duplicates on import with Chrome
- Distinguish between variants
- Refactor file handling for CSV import (create FileReader only once)
- Clear file input field after import
- Fix error handling (hide load mask)
- Improve localization
- Bump Papa Parse dependency